1. Field of the Invention
The invention concerns the production of liquid hydrocarbons by the hydrogenation of coal whereby, in one working operation, refined products are produced with comparably lower boiling points.
2. Description of Prior Art
The crude coal-oils produced by the sump phase hydrogenation of coal or high-boiling products stemming from coal, such as tars, pitch, etc., require additional processing steps in order to arrive at refined liquid hydrocarbons which are stable in storage. In order to increase the thermal efficiency and the economy of the entire process, it is advantageous to arrange the sump phase hydrogenation and the refining stages in series. This is because the processing parameters (pressure, temperature) required for the refining stage result automatically from the sump phase hydrogenation.
Since, in the refining of crude coal-oils from the sump phase hydrogenation, there also occurs a transformation to lower boiling fractions, it is possible to optimize the entire process with regard to the desired product qualities. For an economical processing operation with a high degree of availability of the entire installation, both the service life as well as the optimum reaction conditions of the solid bed catalyst in the refining stage play a decisive role. The quality of the solvent required for mashing the coal is of importance.
Several procedures are known for the direct arranging in series of sump phase hydrogenation and gas phase hydrogenation (solid bed catalyst) in the production of refined liquid hydrocarbons from coal, products which originate from coal (pitches, tars, etc.) and from heavy oils which stem from petroleum. In the "Combination-Hydrogenation Chamber" by W. Urban (Journal: Erdol und Kohle", 8th year, Nov. 1955, No. 11, pages 780-782), the hydrocarbons, which are produced in the sump phase reactors and which contain low-, medium- and high-boiling point fractions, are conveyed at about 430.degree. C. and about 300 bars over gas phase reactors with a solid bed catalyst filling. This process, which is presently known by the name VEBA-COMBI-CRACKING, was also carried over to coal hydrogenation (Journal: ENERGIE, year 34, No. 6, June 82, pages 172-173). In the combination process of L. Raichle and W. Kronig (German Laid Open patent application No. 26 54 635), the coal-oil vapors produced in the sump phase hydrogenation are divided into two parts. In the process, part is taken over the gas phase reactors with a lump catalyst whereby, after a subsequent separation of liquid and gas, and distillation, the low boiling point fractions are led away as products and the higher boiling point fractions (hydrogenated medium and light oils) form the solvent for the coal-mashing operation. After leaving the sump phase hydrogenating unit (hot precipitator), another fraction is condensed-out directly and provides the required medium oil and heavy oil fractions for mashing the coal. In this way, there is produced a solvent which comprises a mixture of hydrogenated (in the gas phase reactor) and non-hydrogenated medium and heavy oils. In this regard, this process differs from other hydrogenating processes (as for example, the EXXON process) in which the entire solvent is hydrogenated. However, the Raichle and Kronig process is disadvantageous in that the directly condensed-out coal-oil vapors also contain a considerable amount of the light oil produced, which results in an unrefined end product.
All the above-mentioned processes suffer from the drawback that the crude hydrocarbon vapors taken via the gas phase reactors with solid bed catalysts contain a large portion of high boiling oils which lead to increased coke formation and thus reduce the service life of the lumpy catalyst.
It has been suggested that for certain applications, an intermediate precipitator can be utilized and such intermediate precipitators are well known in pure sump phase hydrogenation processes which do not incorporate subsequent gas-phase hydrogenation. In such applications, the intermediate precipitator is located after the hot precipitator. However, in such applications, the intermediate precipitator is so operated with regard to temperature and pressure that there results in the intermediate precipitator sump an amount of solvent which comprises medium and heavy oil and which is necessary so that when the sump is admixed as a partial stream with another partial stream stemming from a vacuum column, the solvent self-sufficiency of the sump phase hydrogenation process is ensured. (U. Bonisch and B. Strobel: German Laid Open patent application No. DE 30 22 158). Because the head products of the intermediate precipitator contain only product oil comprising light and medium oils (and amounts of heavy oil, if such be the case), the otherwise usual distillation phase for separating product and solvent oils becomes superfluous. It is, however, disadvantageous that an amount of light oil which is fed back again in the solvent to the sump phase hydrogenator is also drawn-off in the intermediate precipitator sump.